I think this will get you what you need:

DECLARE @Result varchar(1000);

WITH x(xx) AS (select * from Xml_Export listing for xml auto ,elements ,ROOT('Listings'))

SELECT @Result='<?xml version="1.0" encoding="UTF-8" ?>'+xx

FROM x

--this will include the ?xml declaration

--if you are exporting to an xml file then don't change it to xml

SELECT @Result

--This will strip the declaration

SELECT CAST(@Result AS xml)

I included two select statements in my example code to demonstrate how, when you cast as xml, the xml declaration is stripped out. For export you should declare @Result as varchar.

Edit: Added comments to my code.